1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Related Art
In recent years, nonvolatile memories, such as NAND flash memories, have widely been developed and manufactured. A memory cell transistor of the nonvolatile memory includes a tunnel insulation film formed on a semiconductor substrate, a floating gate electrode formed on the tunnel insulation film, an inter-electrode insulation film formed on the floating gate electrode, and a control gate electrode formed on the inter-electrode insulation film.
In normal cases, since a silicon oxide film is used for the tunnel insulation film, an oxidizing agent easily enters the tunnel insulation film during a fabrication process. As a result, so-called bird's beak oxidation occurs, leading to an increase in operation voltage of the memory cell transistor and degradation in reliability.
It is possible to suppress progress of bird's beak oxidation by providing nitride films on the upper and lower surfaces of the tunnel insulation film. Jpn. Pat. Appln. KOKAI Publication No. 2002-353343, for instance, discloses a structure in which oxinitride films are provided on the upper and lower surfaces of the tunnel insulation film. However, if nitride films are provided, charge trap density and positive fixed charge density increase, leading to degradation in characteristics and reliability.
The above-described problem may similarly occur with respect to the inter-electrode insulation film of the memory cell transistor. Furthermore, the above-described problem may similarly occur with respect to gate insulation films of a peripheral circuit transistor and a select gate transistor in a NAND flash memory.
As has been described above, in the prior art, no effective solution to the problems with the bird's beak oxidation has been proposed, and it has been difficult to obtain a semiconductor device with excellent characteristics and reliability.